In optical communications networks, optical transceiver modules are used to transmit and receive optical signals over optical fibers. On the transmit side of the transceiver module, a laser generates amplitude modulated optical signals that represent data, which are then transmitted over an optical fiber coupled to the transceiver module. Various types of semiconductor lasers are typically used for this purpose, including, for example, Vertical Cavity Surface Emitting Lasers (VCSELs) and edge emitting lasers, which may be further divided into subtypes that include Fabry Perot (FP) and Distributed Feedback (DFB) lasers.
On the receive side of the transceiver module, an optics system of the module focuses light propagating out of the end of an optical fiber onto an optical detector, which converts the optical energy into electrical energy. The optical detector is typically a semiconductor photodiode device, such as a PIN photodiode, for example. Optical transceiver modules typically include one or more laser diodes on the transmit side for transmitting multiple data signals and one or more photodiodes on the receive side for receiving multiple data signals.
In some cases, apparatuses known as transistor outline (TO) cans are used to hermetically package components of optical transceiver modules. The laser diodes, photodiodes and other components are first attached by epoxy or gold-tin eutectic alloys to a header of a receptacle of the transceiver module. The components are then wire bonded to the traces of a printed circuit board (PCB), which functions as the subassembly of the transceiver module. A lid having a transparent window in it is then welded to the header in such a way that the lid encompasses the components on the header in a hermetically-sealed environment. In other cases, the components are placed in a ceramic package and then the package is hermetically sealed by either welding a metalized area of the package to a metal foil, or by glass frit bonding the package to a glass window. While these methods and apparatuses generally provide satisfactory results, they are often costly.
FIG. 1 illustrates a block diagram of the transmit side of a known optical transceiver module 2 having components that are enclosed in a hermetically-sealed environment. The portion of the optical transceiver module shown is typically referred to as a transistor outline (TO)-can transmitter module. The TO-can module comprises a submount assembly 3, a pedestal 4, a laser diode 5 mounted on the pedestal 4, a TO-can lid 6, a lens 7 formed in the lid 6, and electrical leads 8 for providing power and ground and electrical signals between the IC comprising the laser diode 5 and possibly other electrical components housed in the TO-can module, such as one or more photodiodes (not shown) and optical elements (not shown). Typically, one or more optical elements (e.g., lenses, reflectors, etc.) are mounted to the submount assembly 3 for directing light output from the laser diode 5 onto the lens 7, and possibly onto other light-sensitive elements, such as a monitoring photodiode (not shown). The submount assembly 3 is typically made of silicon or of a ceramic material.
The process for attaching the components shown in FIG. 1 and for hermetically sealing them within the lid 6 is as follows. The laser diode 5 and any other components to be placed in a hermetically sealed enclosure (e.g., photodiodes, thermistors, etc.) are attached to the top surface 9 of the submount assembly 3 using a gold-tin die attach process. The optical components such as any lenses or reflectors (not shown) are attached to the top surface 9 of the submount assembly 3 with non-electrically conducting epoxies. Electrical components such as thermistors (not shown) and decoupling capacitors (not shown) are attached to the top surface 9 of the submount assembly 3 using a conductive epoxy.
To provide a hermetic seal for this enclosure, a solder ring disposed on the bottom surface of the lid 6 is welded to the top surface 9 of the submount assembly 3 during a solder flowing process. When the solder is cooled, it hardens, fixing the lid 6 in place and forming a hermetic seal between the bottom surface of the lid 6 and the top surface 9 of the submount assembly 3.
Although this hermetic seal normally works well, a few problems may occur. First of all, there is no mechanism for holding the lid 6 in place during the reflow process. Consequently, a small vibration may cause the lid 6 to shift in position, resulting in non-wetting of the top surface 9 of the submount assembly 3 by the solder disposed on the bottom surface of the lid 6. This may result in the seal failing. Second, a shift in the position of the lid 6 may result in the lid 6 damaging wire bonds on the submount assembly 3, which may result in components not operating properly. Furthermore, the lid attachment temperature is limited on the upper and lower ends of a temperature range, which can result in other difficulties that must be addressed. On the upper end, the attachment temperature is bounded by the melting temperature of the gold-tin eutectic and by the degradation temperature of epoxy. On the lower end, the attachment temperature is bounded by the melting point of the flex circuit attachment. Having to work within these bounds can result in the occurrence of other problems.
Accordingly, a need exists for ways to hermetically seal the components of the optical transceiver module within a hermetically-sealed environment that are relatively inexpensive, that ensure that the lid is positioned to achieve adequate wetting of the lid and submount assembly with solder, and that ensure that the lid is held in place during the reflow process so that it cannot shift in position.